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Battery Management - Chargers Forum
BQ25504 Verification and How to get more VLOAD Current
I recently had an opportunity to breadboard the BQ25504 using an adapter to get to the ultra small pins. Essentially implemented the solar charger circuit in the data sheet with exception that I set up the resistors for a 3.7V LiPoly single cell battery. Hooked up t a 4 volt open circuit solar panel and the battery. I started to probe around on the various voltage sense pins to verify it was working. The voltages at the pins are varying as the part is sampling the pins. I set up the BAT_UV to 2.85V and the BAT_OV for 4.2V. Also the VBAT_OK is 3V for the discharge and 4V for the charge. I have a large load on the device (USB Supply) and this may be the undoing. Each time the load is switched in the voltage droops at the VSTOR pin and the UV and OK thresholds are exceeded. Of course the VSTOR is turned off so then my load is powered off.
The spec sheet seems to imply 2 ohms of resistance @ 50mA between the BAT and LOAD pins. 2 Ohms seems to be to high for my application.
I have tried to move the load to the battery side and adding extra cap to the battery side as suggested. That is working for now but completely eliminates the VLOAD cutoff to protect the batteries from undervoltage. I tried to use an external power FET driven from VLOAD to switch in the higher current load but that was not working either. The power transistor I tried needed at least 4V at the gate to supply the 500mA required. I may try a few other devices but the bottom line is that the BQ25504 VLOAD switch is way undersized for a heavier load. May need to go low tech and try a small relay powered from the vload side. Just trying to gain back battery protection with the switching of VLOAD.
Any suggestions on increasing the current output on the BQ25504 VLOAD pin?
The bq25504 is a low power device and the harvesting input has to be sized for the load and kept within the power limitations of the boost converter.
There is a peak power limitation of 400mW which is 100mA at 4V.
Also there is a limitation on how low one can set the UV (>2.2V).
You could short the VSTOR and OUT together which makes the UV threshold meaningless, but now as you pointed out you can't disconnect the battery.
One alternative thought, would be to use the VBAT_OK output to drive an external FET.
So, when VBAT_OK goes high you invert it, and turn on a PMOS switch from VBAT to your load.
This would give even more flexibility, sinc eyou can program both the turn on and off. Versus the internal FET is only switch with UV and a fixed hysteresis.
This would give even more flexibility, since you can program both the turn on and off. Versus the internal FET is only switch with UV and a fixed hysteresis.
Thanks guys. I did manage to find a few fet switches with ultra low Rds_on and I tried to hook up the load through one. It seems to be working at the moment so that is good. My bigger issue is just the small sized solar panel used to harvest energy. Only puts out about 20mA peak. If my math is correct it could take a while to completely charge my battery. So I need to find a more appropriate sized panel and give that a try. I didn't catch the UV setting had to be higher than 3.2V earlier. Adjusted resistor values and it's now set for 3.2V. Completely missed that one. Thanks all.
I double checked the UV setting again and the limit in the spec sheet is 2.2V so my 2.8V should have been OK but I admit taking a LiPo to 2.8V is not a great idea. It was a fail safe setting only. I'll keep it at 3.2V for now and see how it performs with a higher current cell attached.
The minimum UV setting is 2.2V
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